1. Field of the Invention
The present invention relates to a method of manufacturing electro-optical devices and, more particularly to a method of manufacturing electro-optical liquid crystal devices of light-weight thin panel type.
2. Description of the Prior Art
There have been well known in the art several liquid crystal displays utilizing super-twisted nematic liquid crystals or twisted nematic liquid crystals cooperative with thin film transistors for use in microcomputers, wordprocessors and the like. Other liquid crystal devices include ferroelectric liquid crystal displays proposed recently. These conventional devices comprise a liquid crystal material disposed and supported between a pair of substrates. The inner surfaces of the substrates are provided with electrode arrangements in order to define a plurality of pixels. The optical property of the liquid crystal material corresponding to a respective pixel is modulated by selectively applying an electric field to rearrange the liquid crystal molecules having an anisotropical dielectric.
In the case of a twisted nematic liquid crystal device as schematically illustrated in FIG. 1, a first substrate 201 is provided with a first electrode arrangement 202 made of indium tin oxide. The inner surface of the substrate 201 is coated with polyimide film 203 over the first electrode arrangement 202. An orientation control surface is formed by rubbing the polyimide film 203. A second substrate is prepared in the same manner. The first and second substrates are joined in order that the rubbing directions are perpendicular to each other. When a twisted nematic liquid crystal material is disposed between the first and second substrates, the liquid crystal molecules are aligned parallel to the rubbing directions at the both surfaces of the substrates so that a helix is formed between the substrates with a minimal energy condition of the liquid crystal molecules.
Such conventional liquid crystal devices require polarizing plates for optical operation and the liquid crystal molecules have to be arranged in a prescribed orientation in order to utilize a desired optical anisotropy.
On the other hand, there have been also well known in the art other type liquid crystal displays utilizing dispersion of liquid crystal materials. A nematic, cholesteric or smectic liquid crystal material is dispersed in and supported by a solid state transparent polymer, e.g. in the form of granules as supported in a sponge-like structure. Namely, a liquid crystal material is encapsulated in a polymer by dispersion and laminated on a substrate in the form of a thin film. Examples of such polymers include gelatin, gum arabic or polyvinyl alcohol.
The polymer and the liquid crystal material are selected to have an equal dielectric anisotropy when the liquid crystal molecules are aligned in a certain direction by virtue of an electric field applied thereto. By this structure, the thin film becomes transparent when such an electric field is applied and becomes opaque when the electric field is removed since the liquid crystal molecules are oriented to various directions so that incident light is scattered. Other examples of this type displays are such utilizing liquid crystal capsules dispersed in an epoxy resin, phase separation between a liquid crystal material and a photo-curing resin and a three-dimmensionally chained polymer impregnated with a suitable liquid crystal material. In this description of the present invention, such encapsulated liquid crystals are called dispersion-type liquid crystals in general.
There is, however, a need for such liquid crystal displays to manufacture large display panels at low costs without complicated processes. One of large problems of manufacturing large panels is difficulty in charging a liquid crystal material between a pair of substrates in order to align the liquid crystal molecules. FIG. 2 illustrates a prior art technique of charging a liquid crystal material into a narrow space formed between a pair of substrates. After liquid crystal display 301 is placed in a vacuum chamber, the vacuum chamber is evacuated by a vacuum pump to remove air from the inner space of the display. An inlet port 302 of the liquid crystal display is then immersed in a liquid crystal material 303 contained in a reservoir in the vacuum chamber. The liquid crystal material enters the inner space by a differential pressure by leaking nitrogen into the vacuum chamber to elevate the pressure of the chamber.
In the case of A4 panels, the distance between the inlet port 302 and the end of the inner space reaches 200 mm or longer. The distance of the substrates, i.e. the thickness of the space does usually not exceed 20 micrometers. The liquid crystal material have therefore to travel such a narrow space for 200 mm or longer distance so that the charging time becomes very long.
Also, the liquid crystal display becomes eventually expensive due to an expensive manufacturing system including an evacuation device. Furthermore, the liquid crystal material is prepared by blending a number of liquid crystals having different transition temperatures and vapor pressures so that the optical characteristic of the liquid crystal material may possibly be modified because composition of the blend is changed by partial evaporation of a constituent liquid crystal having a high vapor pressure due to exposure to vacuum. Still further, the weight of the display per unit area is increased as the size of the display increases since the thickness of the substrate must be increased in order to maintain the distance between the substrates at a constant level. This contradicts the general current demands for small and light weight devices.